<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>

  <style>
    .box {
      width: 100px;
      height: 100px;
      background-color: red;
      /* 外轮廓是不占位置的 */
      outline: 20px green solid;
    }

    a {
      /* 行内级元素设置上下margin是无效的 */
      display: inline-block;
      margin-top: 100px;
      /* 将链接的外轮廓去除 */
      outline: none;
    }
  </style>
</head>

<body>
  <div class="box"></div>
  <a href="#">百度一下</a>
</body>

</html>